Antenna arrays are used for wireless transmission to provide enhanced data-rates or increased reliability of reception compared to single antenna transmission. There are a wide variety of transmission techniques that can be used with an array of antennas at a transmitter in a wireless system. One example is transmit beamforming (sometimes referred to as transmit adaptive array (TXAA) transmission). Transmit beamforming increases the effective signal-to-noise ratio seen by a receiver device by creating a coverage pattern that tends to be directional in nature (i.e., not uniformly broadcast). This is accomplished by weighting each antenna such that the combined transmissions result in a beamformed pattern having a maximum power in the direction of the receiver. Transmit beamforming can be deployed by a base station operating in cellular communication systems and generally requires some knowledge regarding the channel response between the antenna array and the intended receiver.
Despite the advantages of transmit beamforming, there are circumstances where it is desirable for a base station to transmit data without using transmit beamforming. For example, in wireless communication systems, a base station may transmit broadcast control information that is intended to be received by any subscriber device that happens to be in the coverage area of the base station. Such control information must in general be transmitted with a transmission pattern that is uniform over the coverage area of the base station. Other examples arise when the transmitted data is intended to be received by multiple users simultaneously, and consequently the transmission cannot be customized for any particular user. Another situation may occur when the information about the wireless channel available at the transmitter is not suitable for customizing the transmitted signals to a particular user, for example, a cellular communication system where the mobile receivers have high velocity and the transmitter base-station does not have up-to-date channel information for the mobiles. In cases where transmit beamforming is not appropriate, there are a variety of space-time-coding and transmit diversity techniques that can be employed by the transmitter, but these techniques generally require the receivers to be configured specially for the particular transmit technique.
When transmit beamforming is not appropriate, there exist situations where the transmitted data must appear indistinguishable from a single-antenna transmission to avoid the need for the receivers to be cognizant of the specific transmission scheme. In such scenarios, the base station can simply transmit with only one transmit antenna. If, however, Power Amplifiers (PA's) with a limited linear operating region are deployed behind the transmit antennas, the transmitter cannot simply increase the transmit power fed to one transmit antenna to match the total power that could have been transmitted had all the transmit antennas been exploited. As a result, transmitting with only one antenna would result in a significant loss in transmit power (⅞ of the power is lost with 8 transmit antennas, ¾ of the power is lost for 4 transmit antennas, etc.). Moreover, the spatial diversity of the transmit antenna array is lost with single-antenna transmission. On the other hand, sending the same waveform to all transmit antennas causes the effective transmit antenna pattern to have nulls in various fixed locations in the coverage area, which is unacceptable for transmissions requiring a uniform coverage pattern.
In such situations where a transmit antenna array must provide a uniform coverage pattern while transmitting from all antennas, delay diversity or cyclic-delay diversity is a viable transmission method. In particular, an orthogonal frequency division multiplexing (OFDM)-based system may employ cyclic shift diversity (CSD) to provide a transmit array pattern that avoids spatial nulls and may be broadcast in nature while not requiring the receivers to be explicitly aware of (or configured for) CSD.
In a multi-carrier communication system such as OFDM, CSD may be applied at the transmitter by first calculating the inverse fast Fourier transform (IFFT) of the frequency domain signal. The signal transmitted by each antenna may then be created by performing a circular shift of the resulting signal by an integer multiple of a basic integer shift and then adding the cyclic prefix. While the effective transmit antenna pattern no longer has spatial nulls, the effective channel seen by a receiver may unfortunately contain nulls in the frequency domain due to the application of CSD. While CSD will produce a performance gain relative to a single antenna transmission in many cases, in certain channels there will be a performance loss due to the presence of the frequency response nulls created by the application of CSD. Such channels include those that are strongly Rician. Therefore, there is a need for a method and apparatus which provides the advantages of CSD, but which mitigates the performance loss in certain channels.